The high state of development of new large electrolytic cells, reflected above all in low cell voltages, high current and energy yields, ease of operation and operating safety of electrolysis plants, is due to a number of measures and improvements relating, not in the least, to the electrode.
Technical anode materials must meet a number of specifications including, among others, the corrosion resistance of the anode material and the progress of the anode process with a sufficiently high speed and the least possible excess voltage. Anode materials used heretofore on a large industrial scale meet these constantly increasing demands only partially. For example, there is a certain amount of unavoidable burning off when graphite anodes are used. In modern large cells, this requires expensive equipment for the maintenance of a constant spacing between the anode and the cathode, in addition to which a relatively high expense is necessary for brine cleaning.
In addition to graphite anodes, anodes of precious metal, such as platinum, metals of the platinum group, and their alloys also have been used. Such anodes always have the disadvantage of very high investment costs and of a relatively heavy wear of noble or precious metal. Anodes of platinized titanium have recently become known, mainly for price reasons, but they have always failed in the sector of Hg electrolysis for reasons of their great amalgam sensitivity.
The expression "valve metals" has lately become very popular for the group of metals including titanium, tantalum, niobium, zirconium, tungsten and molybdenum. It is known that these valve metals passivate very quickly when used in aqueous solutions, due to the development of a dense cover layer of an oxidic nature, thereby becoming extremely corrosion-resistant in many electrolyes. However, the passive layers of these metals have no electron conductivity in the electric potential ranges here in question, so that very high field densities occur in the layers. Above a certain potential, called "breakthrough potential", this leads to the destruction of the passivating layers. Despite the fact that these metals have great corrosion resistance, no anode process can be carried out with these metals in the passive state. It is usually not noted that, even in the noble or precious metals, the Flade potential, which is the potential at which the metal passes over from the active to the passive state, is considerably more negative than the normal potential. Accordingly, at higher potentials the noble metals also are covered by passive layers in electrolytes. In platinum, a monomolecular oxygen-chemisorption layer on the metal surface will already lead to passivity. It is immaterial, for this passive layer mechanism, whether the cover layer of an oxidic nature is generated on the noble metal in the electrolyte or whether oxidic noble metal cover layers are applied prior to immersion in the electrolyte, as proposed for the dimensional stable anodes in DT-OS 18 14 567 (German patent application laid open for publicatin without examination). These passive layers on noble metal bases, in contrast to the passive layers of the valve metals, distinguish themselves by their good electron conductivity, thus permitting carrying out of the anode process.
It is obvious, however, that the anchoring of foreign substances to the carrier metal, such as cubic-face-centered platinum to titanium which, at the temperatures used, is most densely compacted hexagonally as a rule, is problematical. Also, the mechanical durability of oxide layers adhering to metal is unsatisfactory because, due to the difference in contractional behavior under rapid temperature changes, stresses will develop in the boundary area between the oxide and the metal, and these stresses cause the oxide to flake off, as is clearly demonstrated by specimens which have been oxidized for some time in air at elevated temperatures. As is known, this method of rapid temperature change is also employed frequently, in industry, for the removal of scale layers. This should also explain adequately the susceptibility of the anodes, according to DT-OS 18 14 567 (German patent application laid open for publicatin without examination), which are provided with ceramic semiconductor coatings, and in which the active cover layer, provided with a chlorine releasing catalyst rests on the bare or on the oxide layer covering the valve metal base.